Protective coating of silver

ABSTRACT

In the method, silver is protected against tarnishing using an Atomic Layer Deposition method. In the Atomic Layer Deposition method, a thin film coating is formed on the surface of silver by depositing successive molecule layers of the coating material. For example aluminium oxide (Al 2 O 3 ) or zirconium oxide may be used as the coating material.

This application is a continuation of application Ser. No. 12/162,051,filed Jul. 24, 2008, which in turn is a National Phase of InternationalApplication No. PCT/FI2007/050056 filed on Jan. 31, 2007, which claimspriority to Finnish Patent Application No. 20065082 filed on Feb. 2,2006. The disclosures of the prior applications are hereby incorporatedby reference herein in their entireties.

FIELD OF THE INVENTION

The present invention relates to a method for coating silver products,and more particularly to a method according to the preamble of claim 1coating silver.

BACKGROUND OF THE INVENTION

Silver naturally tarnishes in the atmosphere, especially in the presenceof sulphur. Industrial atmospheres and natural digestive processes areimportant sources for the tarnishing of silver. When silver istarnished, sulphides, oxides or carbonates are formed on the surface ofsilver. The tarnishing of silver and silverware is a problem for examplefor the utility article, jewellery and giftware industries as well as tothe end users of the silver products. Tarnishing degrades the appearanceof the product as a layer or spots of black or dark grey colour areformed. It can be removed but this is usually a laborious process, andthe process may affect the appearance of the product negatively. Also intechnical applications the tarnishing of the silver reduces the opticalproperties, such as reflectivity, of silver and silver products andparts.

Methods for preventing tarnishing in advance are known in the prior art.One existing method for preventing tarnishing comprises using silveralloys that are designed with resistance to oxidation and involve themixing of special additives, such as silicon or germanium, with silver.Another existing method for preventing tarnishing comprises coating puresilver using rhodium. One of the problems associated with the abovearrangement using silver alloys is that the method requires all factorsto be carefully controlled during manufacture, like using extremely purenew metal and accurate temperature control in melting and annealing. Asa result, the manufacturing process and the equipment to carry out theprocess are very expensive to set up. The cost is also prohibitive inthe method for coating silver with rhodium. Further, the rhodium coatinghas a blue-white shade, and thus a silver product coated with rhodiummay become visually different from pure silver.

Other existing methods for preventing tarnishing of silver comprisecoating the finished silver product, article or part with a method thatprovides a layer of material on the silver product, article or partwhich prevents or restrains tarnishing on the finished silver product,article or part. These kind of prior art methods comprise varnishing thesilver products. The problem with these known coating methods is thatthe coating layer is not uniform over the whole product or part of theproduct that has been coated. The thickness variations in the coatinglayers over the silver product cause colour variations, for example dueto interference, or other optical alterations, which are not preferable.These known methods also produce relatively thick layers of coatingmaterials on the silver products. This further has a negative influenceon the appearance of a silver product. Varnish may also yellow and peeloff. Thus, the known methods for preventing tarnishing of a silverproduct do not provide a uniform and substantially to human eyeinvisible coating, but a non-uniform coating and/or a coating producingdiscoloration of a silver product.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present application is to provide a method so as toalleviate the above disadvantages. The objects of the application areachieved by a method according to the characterizing portion of claim 1.Therefore the present invention is characterized by applying a thincoating of protective material on at least a part of the surface of asilver product, article or part using an ALD (Atomic Layer Deposition)method.

Preferred embodiments are disclosed in the dependent claims.

The term thin layer means in this context a layer having thicknessbetween 1 nm and 1 μm, preferably between 1 and 100 nm, and mostpreferably approximately 2-20 nm.

In the method a thin film coating is deposited on the surface of asilver object. In the present solution, silver is coated with one ormore molecule layers of aluminium oxide Al₂O₃. Trimethyl aluminium(CH₃)₃Al may be used as a precursor and water H₂O as an oxygen source.The thickness of the generated thin film per one ALD cycle is about 0,1nm and the coating is carried out in a temperature of about 200° C.

In experiments desired results have been achieved by depositing about 3nm coating of Al₂O₃ on a silver product by using 30 ALD cycles withsuccessive pulses of trimethyl aluminium (TMA) and water. Thickercoatings have also been tested to determine the relation between thecolor of the coating and the thickness of the coating. Another goodthickness range is found be about 70 nm range. This thickness may beachieved by depositing about 70 nm coating of Al₂O₃ on a silver productby using 700 ALD cycles with successive pulses of TMA and water. Anadvantage of the present solution is that it is possible to produce athin coating, which effectively prevents silver from tarnishing withoutaltering the appearance of the silver product. Also the opticalproperties of the silver will remain substantially unaltered. Thus thecoating passivates the silver surface. The coating generated by themethod is thin, dense, smooth and substantially colourless, and itprecisely follows conformally the shapes, also three dimensional shapes,of the silver object without thickness variations in the coating. Bymeans of the present solution, a stable, uniform and attractive coatingmay be achieved. The generated coating is compatible with foodstuffs.The consumption of the coating material is low, and thus coating costsmay be reduced. The thickness of the coating layer may be controlled byvarying the number of molecule layers in the coating. The coatingprocess is not sensitive to minor changes in the process parameters, andthus the repeatability of the method is good. This thin layer issufficient for preventing the tarnishing of the silver, but does notaffect the appearance of the silver product, as the conventional coatingmethods. The coating may be so thin that human eye cannot se it. Such auniform layer is not possible to be provided on a three dimensionalobject for example with CVD method (Chemical Vapour Deposition) or PVD(Physical Vapour Deposition) method, since the coating process may notbe controlled in such a detail as with ALD method. CVD and other similarmethods also require that the coated object have to be rotated forproviding coating material over the whole surface of the threedimensional object.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail bymeans of preferred embodiments with reference to the accompanyingdrawings, in which

FIG. 1 is a schematic representation of the coating process of silverwith aluminium oxide according to the solution of the presentapplication;

FIG. 2 is a schematic representation of the structure of the coatingaccording to the solution of the present application.

DETAILED DESCRIPTION OF THE INVENTION

Silver naturally tarnishes in the atmosphere, especially in the presenceof sulphur. Industrial atmospheres and natural digestive processes areimportant sources for the tarnishing of silver. When silver istarnished, sulphides, oxides or carbonates are formed on the surface ofsilver. Tarnishing degrades the appearance of the product as a layer orspots of black or dark grey colour are formed. Also in technicalapplications the tarnishing of the silver reduces the opticalproperties, such as reflectivity, of silver and silver products andparts. For preventing the tarnishing of surfaces of a silver product athin coating may be provided on the surfaces of the silver product. Thecoating should be sufficiently thin for preventing the change of theappearance of the silver product, but sufficiently thick to provide goodpassivation and/or protection against tarnishing. This kind of thincoating may be applied on the surfaces of a silver product preferably byusing Atomic Layer Deposition (ALD).

Atomic Layer Deposition is a thin film technique that allows thin filmcoatings that have a nanoscale thickness to be manufactured. ALDtechnique may also be called ALC (Atomic Layer Coating) technique or ALE(Atomic Layer Epitaxy) technique. The ALD is based on a gas phaseprocess where primary compounds are typically evaporated and pulsed intoa reaction chamber separately. A thin film is generated when thematerial obtained from the reaction between the primary compounds isdeposited on the surface to be coated. The material is deposited on thesurface such that successive layers of molecule level are deposited oneby one. This may be called “growing” of the material. Thin filmmaterials obtained by means of the ALD technique include, for example,metal oxides and metal nitrides.

In the Atomic Layer Deposition method, a thin film coating is formed onthe surface of silver by depositing successive molecule layers of one ormore coating materials.

According to the solution of the present application, the ALD techniqueis adapted to coating an object comprising silver. In the presentsolution, the object comprising silver is coated with a coatingcomprising aluminium oxide Al₂O₃. However, any colourless metal oxide,such as zirconium oxide ZrO₂, titanium oxide TiO₂, chromium oxide Cr₂O₃,indium oxide In₂O₃, niobium oxide Nb₂O₅, or any other materialobtainable by the ALD technique may also be used.

FIG. 1 illustrates an embodiment of the present solution showing thecoating of a silver surface S with aluminium oxide Al₂O₃. The coating isbuilt up of molecule layers of aluminium oxide. FIG. 1 shows a situationwhere trimethyl aluminium (CH₃)₃Al and water H₂O are used as primarymaterials. If silver were coated with ZrO₂, for example, ZrCl₄ and H₂Omight be used as primary materials.

In step 1-1, the surface S is exposed to gas comprising trimethylaluminium, in which case a layer of trimethyl aluminium molecules(CH₃)₃Al is formed on the surface S. In step 1-2, the residual gas hasbeen removed, and the layer comprising trimethyl aluminium molecules(CH₃)₃Al remains on the surface S. In step 1-3, the surface S hasfurther been exposed to water H₂O. In the reaction between trimethylaluminium (CH₃)₃Al and water, aluminium oxide Al₂O₃ is formed. Thereaction proceeds stepwise, and also other compounds may be formed, suchas aluminium hydroxide AlOH and methane CH₄. During the reaction,aluminium oxide Al₂O₃ is deposited on the surface S. Step 1-4 shows asituation where unreacted trimethyl aluminium (CH₃)₃Al and eventualother compounds have been removed, and there is a layer of aluminiumoxide Al₂O₃ deposited on the surface S.

The thin film coating of the present solution is obtained by growing ofthe material. This is carried out by repeating the steps 1-1 to 1-4 ofFIG. 1 several times such that successive layers of aluminium oxidemolecules are deposited on the surface S. The thickness of the coatingmay be controlled by varying the number of molecule layers.

FIG. 2 illustrates a situation where the coating on the surface S ofsilver comprises four molecule layers of aluminium oxide Al₂O₃. Inreality, the number of successive layers of aluminium oxide Al₂O₃ may beother than four.

In the coating process, usually a coating that is as thin as possible isdesirable such that it will still be sufficiently thick in order to havethe desired properties. According to the present solution, the thicknessof the coating is preferably within the range of 1 nanometre to 1micrometer, more preferably within the range of 5 to 200 nanometres,most preferably about 10 nanometres. The thickness of the coating may beadjusted by varying the number of the molecule layers of the coatingmaterial.

Experiments have shown, that the one preferable thickness range isbetween 1 and 15 nm. The yellowish appearance increases as the thicknessof the coating increases (in range 0 to 50 nm) being very disturbing asthe thickness of the coating is 20 nm or more. On the other hand theprotective and/or passivating effect of the coating gets better as thethickness of the coating increases. Therefore, the thickness of thecoating should be a compromise between the protection against tarnishingand the appearance of the silver product.

Another good thickness range is reached when the interference of thethin coating starts to intensify the blue color. Thus the blue coloreffect produced by the coating makes it brighter to the human eye. Thiskind of effect may be produced with aluminum oxide deposited with ALD ona silver product when the thickness of the coating is in range about 60to 90 nm. In this case also the passivation and/or protective effect ofthe coating is good. The blue interference appears as a series also whenthe thickness of the coating is increased, but then also other colorsmay appear depending on the angle of view of the product and furthermorethe processing of the silver product becomes slower and more expensive.

The above mentioned effect may also be achieved with other colourless orsubstantially colourless materials or coatings in addition to aluminiumoxide. The preferable thickness ranges vary according to material,because of the differences in the refractive index of the materials.

The lower of the mentioned thickness ranges (1-15 nm) requires abilityto make very uniform and thin coatings as well as a low refractive indexfor the coating material. This is because of the fact that the yellowishappearance becomes a dominant feature before a sufficient thickness forthe passivation and protection is achieved when materials having highrefractive index are used. Aluminium oxide has proved to be one suitablematerial for providing thin uniform coatings to prevent tarnishing of asilver product when ALD is used. For example, when zirconium oxide isused, the appearance of a silver product becomes too yellowish beforethe sufficient passivation level is reached. A thin coating ispreferable for the silver product. Then a material having low refractiveindex may be used. On the other hand, if excellent passivation isrequired and thus the thickness of the coating must be increased. Inthis case a material having higher refractive index may be used forachieving desired result, since the desired result may be achieved withthinner coating compared to material having lower refractive index.

The solution of the present application is based on the idea ofprotecting silver against tarnish by coating it using ALD (Atomic LayerDeposition) method. ALD is suitable for a precise production of verythin coatings. As a method ALD is also very suitable for commercialproduction requirements. The scalability and versatility of ALD makes anattractive method for producing coatings in industrial production.

Experiments have shown that the growth produced by ALD starts as columnsand not until the coating is approximately 3 nm in thickness it issufficiently uniform and unbroken for preventing the surface of a silverproduct from tarnishing. On the other hand, when aluminium oxides areused as coatings, the surface of a silver product starts to lookyellowish already when the thickness of the coating is 10 nm. Therefore,as optically more coarse material is chosen the thickness of the coatingmay be larger, but when optically more dense material is chosen thethickness of the coating should be decreased for preventing theyellowish appearance of the product. Some of the materials may evenproduce a yellowish appearance for the product when the thickness of thecoating is not even sufficient for producing a uniform and unbrokencoating. Accordingly, the thickness of the substantially invisiblecoating produced by ALD may differ depending on the materials used suchthat the coating on a silver product is sufficient for producing auniform and unbroken coating, but sufficiently thin for preventingdiscoloration of a silver product. Here, the optical dense isinfluenced, in addition to the refraction index, by reflectance factors,boundaries, imaginary components etc.

The temperature used in the coating process depends on the materialproperties. In many cases, it is advantageous to use a relatively hightemperature. A high temperature allows molecules to evaporate readily,and a coating having a sufficiently good quality is obtained. Accordingto the present solution, the coating temperature is preferably withinthe range of 80 to 400° C., more preferably within the range of 120 to300° C., most preferably about 200° C.

Aluminum oxide process functions at least in a temperature range 100 to250° C., and part even in a temperature range 20 to 300° C. Theserelatively low temperature ranges enable the coating for preventingtarnishing to be deposited after possible gemstones are planted and/orassembly solder joints or other assembly steps are conducted. Thus, allsurfaces may be protected and there is no need to touch the surfaceswith tools. The low temperature also enables rapid processing of thesilver products using these low temperatures, which also makes treatmentmore simple and advantageous.

According to another embodiment of the present solution only a part ofan object or a surface is coated.

According to yet another embodiment of the present solution, the methodis applied together with one or more protecting methods other than themethod described herein. In that case, the use of silver alloys that areresistant to oxidation, for example, may be applied.

According to yet another embodiment of the present solution, the methodis applied to coating silver-plated objects.

According to yet another embodiment of the present solution, the methodis applied to coating silver alloys.

According to yet another embodiment of the present solution, the methodis applied to coating objects or surfaces comprising bronze, copperand/or brass. In other words this same method may be used to coat alsoother metals so that the appearance of the metal is not influenced.

The method enables objects of various shapes to be coated. Thus it maybe applied to coating jewels, ornaments, tableware, etc., as well asvarious industrial components.

It should be noted that the use of aluminium oxide is not necessarilyrequired by the present solution; any other coating material obtainableby the ALD technique, such as titanium oxide (TiO₂), tantalum oxide(Ta₂O₅) and/or zirconium oxide (ZrO₂), may also be used. Differentcoating materials may be used simultaneously. The obtained coatingshould have the desired properties and it should be compatible with themetal to be coated such as silver. Instead of (CH₃)Al, also othercompounds may be used as precursors, such as aluminium chloride AlCl₃and/or triethyl aluminium (CH₃CH₂)₃Al. Instead of water, also othercompounds, such as hydrogen peroxide H₂O₂, ozone O₃, etc may be used asthe oxygen source. The choice of the coating material may depend on theapplication. For example, tableware or jewellery may require abiocompatible coating layer. An example of a biocompatible coatingmaterial is aluminium oxide Al₂O₃. Reactions illustrated in FIG. 1 mayoccur in different order and also other reactions and/or steps may becarried out.

It is also possible to provide a coating having nanolaminate structureusing ALD with two or more different coating materials. Then protectivematerial is applied on the surface of a silver product such that one ormore successive molecule layers are deposited and then anotherprotective material is applied on the surface of a silver product suchthat one or more successive molecule layers are deposited. This may becontinued until a predetermined coating thickness is achieved. It isalso possible use three or more different materials in the mentionedsuccessive manner. This provides a coating comprising two or more layersof two or more protective materials.

The applied coating is generally so thin that it is invisible to humaneye. The method may therefore be applied to silver jewellery, coins,medals, tableware, ornaments or the like silver products. The method mayalso be applied to products comprising several different materials inaddition to silver. Furthermore, the method according to the presentinvention may be applied to at least part of an electronic or electricalcomponent or other industrial component made of silver or silver alloys.

It will be obvious to a person skilled in the art that as technologyadvances, the inventive concept can be implemented in various ways. Theinvention and its embodiments are not limited to the examples describedabove but may vary within the scope of the claims.

1. A method for protecting silver products, articles or parts againsttarnish in an atmosphere, the method consisting of applying a thincoating layer of protective material having a thickness between 2 nm to20 nm on at least a part of an outer surface of a silver product,article or part using an ald (atomic layer deposition) method to preventtarnishing of the outer surface of the silver product, article or partin the atmosphere.
 2. The method according to claim 1, wherein applyingthe thin coating layer comprises providing at least one layer of metaloxide using the ALD (Atomic Layer Deposition) method.
 3. The methodaccording to claim 2, wherein the metal oxides comprise aluminum oxideAl₂O₃, titanium oxide TiO₂, chromium oxide Cr₂O₃, zirconium oxide ZrO₂,indium oxide In₂O₃, and niobium oxide Nb₂O₅.
 4. The method according toclaim 1, wherein the thin coating layer is substantially transparentnon-oxide material.
 5. The method according to claim 1, furthercomprising applying a substantially colorless coating layer on at leasta part of the outer surface of the silver product, article or part. 6.The method according to claim 1, further comprising applying said thincoating layer at a temperature, which is in a range of 80 to 400° C. 7.The method according to claim 1, wherein the thin coating layer isproduced on a multiple-part silver product after the product isassembled.
 8. The method according to claim 1, wherein the method isapplied to silver jewelry, coins, medals, tableware, ornaments, silverproducts, or at least a part of an electronic or electrical component orother industrial component made of silver or silver alloys.
 9. Themethod according to claim 1, wherein the method is applied to productscomprising several different materials in addition to silver.
 10. Themethod according to claim 6, comprising applying said thin coating layerat a temperature, which is in a range of 120 to 300° C.
 11. The methodaccording to claim 6, comprising applying said thin coating layer at atemperature, which is 200° C.
 12. A method for protecting productscomprising silver against tarnish in an atmosphere, the methodcomprising applying a thin coating of protective material having athickness between 2 nm to 20 nm on at least a part of a product using anALD (Atomic Layer Deposition) method to prevent tarnishing of the silverof the product in the atmosphere.
 13. A method for protecting silverproducts, articles or parts against tarnish in an atmosphere, the methodcomprising applying a thin coating of protective material having athickness between 2 nm to 20 nm on at least a part of an outer surfaceof a silver product, article or part using an ALD (Atomic LayerDeposition) method to prevent tarnishing of an outer surface of thesilver product, article or part in the atmosphere.
 14. The methodaccording to claim 13, wherein the thin coating of protective materialforms an outer surface of the silver product, article or part to preventtarnishing of the outer surface of the silver product, article or partin the atmosphere.